581 research outputs found
Observing Supernova 1987A with the Refurbished Hubble Space Telescope
Observations with the Hubble Space Telescope (HST), conducted since 1990, now
offer an unprecedented glimpse into fast astrophysical shocks in the young
remnant of supernova 1987A. Comparing observations taken in 2010 using the
refurbished instruments on HST with data taken in 2004, just before the Space
Telescope Imaging Spectrograph failed, we find that the Ly-a and H-a lines from
shock emission continue to brighten, while their maximum velocities continue to
decrease. We observe broad blueshifted Ly-a, which we attribute to resonant
scattering of photons emitted from hotspots on the equatorial ring. We also
detect NV~\lambda\lambda 1239,1243 A line emission, but only to the red of
Ly-A. The profiles of the NV lines differ markedly from that of H-a, suggesting
that the N^{4+} ions are scattered and accelerated by turbulent electromagnetic
fields that isotropize the ions in the collisionless shock.Comment: Science, accepted. Science Express, 02 Sept 2010. 5 figures.
Supporting online material can be found at
http://www.sciencemag.org/cgi/content/full/sci;science.1192134/DC
Understanding the effect of curvature on the magnetization reversal of three-dimensional nanohelices
Comprehending the interaction between geometry and magnetism in
three-dimensional (3D) nanostructures is of importance to understand the
fundamental physics of domain wall (DW) formation and pinning. Here, we use
focused electron beam-induced deposition to fabricate magnetic nanohelices with
increasing helical curvature with height. Using electron tomography and Lorentz
transmission electron microscopy, we reconstruct the 3D structure and
magnetization of the nanohelices. The surface curvature, helical curvature and
torsion of the nanohelices are then quantified from the tomographic
reconstructions. Furthermore, by using the experimental 3D reconstructions as
inputs for micromagnetic simulations we can reveal the influence of surface and
helical curvature on the magnetic reversal mechanism. Hence, we can directly
correlate the magnetic behavior of a 3D nanohelix to its experimental
structure. These results demonstrate how control of geometry in nanohelices can
be utilized in the stabilization of DWs and control of the response of the
nanostructure to applied magnetic fields
Spatial Structure and Collisionless Electron Heating in Balmer-dominated Shocks
Balmer-dominated shocks in supernova remnants (SNRs) produce strong hydrogen
lines with a two-component profile composed of a narrow contribution from cold
upstream hydrogen atoms, and a broad contribution from hydrogen atoms that have
undergone charge transfer reactions with hot protons. Observations of emission
lines from edge-wise shocks in SNRs can constrain the gas velocity and
collisionless electron heating at the shock front. Downstream hydrogen atoms
engage in charge transfer, excitation and ionization reactions, defining an
interaction region called the shock transition zone. The properties of hot
hydrogen atoms produced by charge transfers (called broad neutrals) are
critical for accurately calculating the structure and radiation from the shock
transition zone. This paper is the third in a series describing the kinetic,
fluid and emission properties of Balmer-dominated shocks, and is the first to
properly treat the effect of broad neutral kinetics on shock transition zone
structure. We use our models to extract shock parameters from observations of
Balmer-dominated SNRs. We find that inferred shock velocities and electron
temperatures are lower than those of previous calculations by <10% for v_s<1500
km/s, and by 10-30% for v_s>1500 km/s. This effect is primarily due to the fact
that excitation by proton collisions and charge transfer to excited levels
favor the high speed part of the neutral hydrogen velocity distribution. Our
results have a strong dependence on the ratio of electron to proton
temperatures, \beta=T_e/T_p, which allows us to construct a relation \beta(v_s)
between the temperature ratio and shock velocity. We compare our calculations
to previous results by Ghavamian et al. (2007).Comment: 41 pages, 15 figures, 2 tables. Improved comparison to previous
results, added discussion, and incorporated referee's suggestions. Submitted
to Ap
Optimization of Cutaneous Electrically Mediated Plasmid DNA Delivery Using Novel Electrode
The easy accessibility of skin makes it an excellent target for gene transfer protocols. To take advantage of skin as a target for gene transfer, it is important to establish an efficient and reproducible delivery system. Electroporation is an established technique for enhancing plasmid delivery to many tissues in vivo. A critical component of this technique is the electrode configuration. Electroporation parameters were optimized for transgene expression with minimal tissue damage with a novel electrode. The highest transgene expression and efficiency of individual cell transformation with minimal damage was produced with eight 150 ms pulses at field strength of 100 V/cm. This electrode design offers the potential for easier and more reproducible electrically mediated cutaneous plasmid delivery than the simple electrodes currently commercially available. This electrode can be a valuable tool in determining the applicability of electrically mediated cutaneous gene transfer
Three-dimensional distribution of ejecta in Supernova 1987A at 10 000 days
Due to its proximity, SN 1987A offers a unique opportunity to directly
observe the geometry of a stellar explosion as it unfolds. Here we present
spectral and imaging observations of SN 1987A obtained ~10,000 days after the
explosion with HST/STIS and VLT/SINFONI at optical and near-infrared
wavelengths. These observations allow us to produce the most detailed 3D map of
H-alpha to date, the first 3D maps for [Ca II] \lambda \lambda 7292, 7324, [O
I] \lambda \lambda 6300, 6364 and Mg II \lambda \lambda 9218, 9244, as well as
new maps for [Si I]+[Fe II] 1.644 \mu m and He I 2.058 \mu m. A comparison with
previous observations shows that the [Si I]+[Fe II] flux and morphology have
not changed significantly during the past ten years, providing evidence that it
is powered by 44Ti. The time-evolution of H-alpha shows that it is
predominantly powered by X-rays from the ring, in agreement with previous
findings. All lines that have sufficient signal show a similar large-scale 3D
structure, with a north-south asymmetry that resembles a broken dipole. This
structure correlates with early observations of asymmetries, showing that there
is a global asymmetry that extends from the inner core to the outer envelope.
On smaller scales, the two brightest lines, H-alpha and [Si I]+[Fe II] 1.644
\mu m, show substructures at the level of ~ 200 - 1000 km/s and clear
differences in their 3D geometries. We discuss these results in the context of
explosion models and the properties of dust in the ejecta.Comment: Accepted for publication in Ap
Dust Production and Particle Acceleration in Supernova 1987A Revealed with ALMA
Supernova (SN) explosions are crucial engines driving the evolution of
galaxies by shock heating gas, increasing the metallicity, creating dust, and
accelerating energetic particles. In 2012 we used the Atacama Large
Millimeter/Submillimeter Array to observe SN 1987A, one of the best-observed
supernovae since the invention of the telescope. We present spatially resolved
images at 450um, 870um, 1.4mm, and 2.8mm, an important transition wavelength
range. Longer wavelength emission is dominated by synchrotron radiation from
shock-accelerated particles, shorter wavelengths by emission from the largest
mass of dust measured in a supernova remnant (>0.2Msun). For the first time we
show unambiguously that this dust has formed in the inner ejecta (the cold
remnants of the exploded star's core). The dust emission is concentrated to the
center of the remnant, so the dust has not yet been affected by the shocks. If
a significant fraction survives, and if SN 1987A is typical, supernovae are
important cosmological dust producers.Comment: ApJL accepte
Preliminary Spectral Analysis of the Type II Supernova 1999em
We have calculated fast direct spectral model fits to two early-time spectra
of the Type-II plateau SN 1999em, using the SYNOW synthetic spectrum code. The
first is an extremely early blue optical spectrum and the second a combined HST
and optical spectrum obtained one week later. Spectroscopically this supernova
appears to be a normal Type II and these fits are in excellent agreement with
the observed spectra. Our direct analysis suggests the presence of enhanced
nitrogen. We have further studied these spectra with the full NLTE general
model atmosphere code PHOENIX. While we do not find confirmation for enhanced
nitrogen (nor do we rule it out), we do require enhanced helium. An even more
intriguing possible line identification is complicated Balmer and He I lines,
which we show falls naturally out of the detailed calculations with a shallow
density gradient. We also show that very early spectra such as those presented
here combined with sophisticated spectral modeling allows an independent
estimate of the total reddening to the supernova, since when the spectrum is
very blue, dereddening leads to changes in the blue flux that cannot be
reproduced by altering the ``temperature'' of the emitted radiation. These
results are extremely encouraging since they imply that detailed modeling of
early spectra can shed light on both the abundances and total extinction of SNe
II, the latter improving their utility and reliability as distance indicators.Comment: to appear in ApJ, 2000, 54
The Axially Symmetric Ejecta of Supernova 1987A
Extensive early observations proved that the ejecta of supernova 1987A (SN
1987A) are aspherical. Fifteen years after the supernova explosion, the Hubble
Space Telescope has resolved the rapidly expanding ejecta. The late-time images
and spectroscopy provide a geometrical picture that is consistent with early
observations and suggests a highly structured, axially symmetric geometry. We
present here a new synthesis of the old and new data. We show that the Bochum
event, presumably a clump of Ni, and the late-time image, the locus of
excitation by Ti, are most naturally accounted for by sharing a common
position angle of about 14\degree, the same as the mystery spot and early
speckle data on the ejecta, and that they are both oriented along the axis of
the inner circumstellar ring at 45\degree to the plane of the sky. We also
demonstrate that the polarization represents a prolate geometry with the same
position angle and axis as the early speckle data and the late-time image and
hence that the geometry has been fixed in time and throughout the ejecta. The
Bochum event and the Doppler kinematics of the [Ca II]/[O II] emission in
spatially resolved HST spectra of the ejecta can be consistently integrated
into this geometry. The radioactive clump is deduced to fall approximately
along the axis of the inner circumstellar ring and therefore to be redshifted
in the North whereas the [Ca II]/[O II] 7300 \AA emission is redshifted in the
South. We present a jet-induced model for the explosion and argue that such a
model can account for many of the observed asymmetries. In the jet models, the
oxygen and calcium are not expected to be distributed along the jet, but
primarily in an expanding torus that shares the plane and northern blue shift
of the inner circumstellar ring.Comment: To Appear in Ap
An adjustable law of motion for relativistic spherical shells
A classical and a relativistic law of motion for an advancing shell are
deduced applying the thin layer approximation. A new parameter connected with
the quantity of absorbed matter in the expansion is introduced; this allows of
matching theory and observation.Comment: 15 pages, 10 figures and article in press; Central European Journal
of Physics 201
Photoionized Lines in the X-ray Spectra of SMC X-1
We present a detailed spectral analysis of Chandra/ACIS-S CC mode
observations of the massive X-ray binary system SMC X-1. The system was
observed during both the high and low X-ray states of the roughly 60-day
superorbital period. The continuum spectra during both states are well
represented by a power law with photon index =0.9 and a blackbody of kT
= 0.15keV. The high state spectra are dominated by the continuum and
independent of orbital phase whereas the low state spectra show a strong
orbital dependence as well as line emission from O, Ne, Mg, Fe, and Si. This is
consistent with the states attributed to disk precession: during the high state
X-ray emission is dominated by the compact source which is abrubtly eclipsed
and during the low state the compact object is hidden by the disk and a larger,
less luminous scattering region is responsible for the X-ray emission. A
prominent Ne IX feature places a stringent limit (Log = 2.0-2.5) on the
ionization parameter which constrains the wind dynamics of the system. The Fe
line fluxes are related linearly to the blackbody fluxes indicating that both
originate in the same region or are excited by the same mechanism. There is
evidence for structure in the Fe-line that cannot be fully resolved by the
current observations. The pulse period measured during our observations,
0.70571470.00000027s shows that the uninterrupted spin-up trend of SMC X-1
continues. We discuss the implications of our results for models of SMC X-1.Comment: 31 pages including 10 figures. Accepted for publication in Ap
- …